This invention relates to wire line core drilling apparatus.
In wire-line core drilling, the core or inner tube assembly is dripped down the bore of a drill string to a position just behind the drill bit or crown. The assembly has a circumferential landing shoulder which, when the assembly is correctly positioned in the drill string, seats on a counter-shoulder or landing ring in the core barrel outer tube assembly. Spring-loaded latches on the assembly spring outwardly into an annular recess, known as the latch seat, in the inner surface of the outer tube assembly to anchor the assembly against axial movement in the bore. A liquid is pumped down the drill string to assist the movement of the core tube assembly to the correct position relative to the drill bit.
As drilling proceeds, a core of the drilled material is captured by the core receiving tube of the inner tube assembly. After a specified advance of the drill bit an operator on the surface lowers an overshot assembly down the drill string on the end of a wire line. The overshot assembly has jaws which engage with and lock onto a formation, known as a spear point overshot coupling member, on the upper end of the inner tube assembly. The wire line is then retracted, causing a latch retracting mechanism to pull the latches inwardly to release the inner tube assembly so that it can be pulled to the surface. On surface, the core is removed for geological analysis and the core tube assembly is then dropped down the drill string again, assisted by the liquid flow, in preparation for the taking of a deeper sample.
A major problem with this type of system is the inability of the surface operator to determine when the latches have anchored the inner tube assembly in position. Usually, the operator estimates when the correct position of the inner tube assembly has been attained merely by timing the descent of the assembly in the drill string. If for some reason the inner tube assembly is not properly landed and latched, the core which is obtained may be broken and unsuitable for an accurate analysis thereof to be made. Furthermore, a great deal of time is lost if the inner tube assembly is not latched because core then cannot be retrieved by means of the wire line system. The drill string has to be removed from the hole and possibly a "fishing operation" has to be conducted to recover the core.